An organic light-emitting device comprises two opposite electrodes and a plurality of semiconducting organic thin layers between the electrodes. The organic light-emitting device with this structure operates on the principle of the conversion of electric energy into light in the organic materials, known as organic electroluminescence. Specifically, in the structure in which organic layers are sandwiched between positive and negative electrodes, when voltage is applied between the two electrodes, holes from the positive electrode and electrons from the negative electrode are injected into the organic layers. When the injected holes and electrons meet each other, excitons are formed, and when these excitons decay to the ground state, light is emitted.
In the organic light-emitting devices as described above, light generated in the organic layers is emitted through a light-transmitting electrode, and thus organic light-emitting devices can be classified into top emission type, top emission type and dual emission type. In case of the top or bottom emission type device, one of the two electrodes should be a light-transmitting electrode and in the dual emission type device, both the two electrodes should be light-transmitting electrodes.
Following the publication by Kodak Company that the use of organic layers with multilayer structure allows the devices to be driven at low voltage, many studies have been conducted on the organic light-emitting devices. Recently, color display devices using the organic light-emitting device are commonly applied to the portable telephones.
In addition, recent organic light-emitting devices show rapid improvement of efficiency according as the studies on the use of phosphorescent materials as a substitute for the conventional fluorescent materials are performed. Also, there is an expectation that the organic light-emitting devices will substitute for the conventional illumination devices in the near future.
In order that the organic light-emitting devices are used for illumination, the devices should be driven at higher brightness than that of the conventional color display devices, and also must be capable of maintaining a high brightness like the conventional illumination devices. In order to improve the brightness of the organic light-emitting device sufficiently, light emission should be made at large area, and for this purpose, high driving current should be applied. In addition, in order to maintain constant brightness at large area, high current as described above should be uniformly supplied into the large-area devices.